This invention relates to rotatable turbine blades and more particularly to pinned root blades having friction damping means provided therefor.
Pinned root blades are often used in the first or control stage of high pressure turbine elements. It is believed that pinned root blades have better load carrying capabilities than "Christmas Tree" root blades and "T" root blades when they are subjected to partial admission shocks which often occur at the control stage. Pinned root blades are also easily removed and replaced if such action is required. Axial contact faces on pinned root blades which engage the attached rotor are presently flat, single planes. However, because of tolerances on smoothness, the actual contact surfaces may be anywhere on the assumed contact faces. If the actual contact area occurs at or near the center of rotation of the pinned root, the friction damping effect obtained by the pinned root blade's axial surfaces rubbing against the attached rotor is almost neglibible. Since friction damping is a part of net damping and vibratory stresses in the pinned root blade decrease as the net damping increases, minimizing total blade stress necessitates maximizing the friction damping.
Maximum friction damping occurs when the actual contact area on one axial end of the pinned root blade is located as far as possible from the center of rotation of the blade's root. Thus, it is necessary to be able to predict the center of rotation of the blade's root. Additionally, the frictional contact needs only be on the blade root's low pressure side since rotatable blades in axial flow turbines experience an axial force which is toward the axially low pressure side of the blade.
Presently, because the actual contact area is unknown, high blade stress due to low friction damping may cause the blade to fail or behave in an unpredictable manner due to the unknown distribution of the actual contacting surfaces.